The present invention relates to a cap for a prosthetic implant particularly a prosthetic acetabulum implant and to an introduction tool for use therewith.
The replacement of all or a part of the bone surface of an articulation with a prosthetic implant has become a common surgical procedure. The procedure requires the surgeon to exercise both precision and delicacy in the placement of the prosthetic implant. However, it is frequently necessary for the surgeon also to exercise a degree of force, sometimes a vigorous force, in order to situate the prosthetic implant in the desired location on the bone surface.
For example, in an operation to insert a prosthetic acetabulum in a patient's pelvis the surgeon first uses a reamer to grind a cavity of appropriate size in the patient's pelvis. An acetabular cup is then inserted into the cavity. By “appropriate size” is meant a size which is selected by the surgeon as being the most appropriate for that particular patient. Normally, it is desirable to retain as much of the original healthy bone surface as possible.
Commercially available acetabular cups are sold in a range of sizes to suit the needs of individual patients. Generally, acetabular cups are available in sizes of from 42 mm to 62 mm diameter, with 2 mm increments between neighboring sizes.
There are a number of different types of prosthetic acetabular cups. One type of cup is made from polyethylene. These are generally cemented into the acetabulum and require only light pressure to seat them in the cement. One alternative cup type has a polyethylene liner unit for articulation with the femur and a metal shell for insertion into the pelvic cavity. These cups with metal shells may be implanted without cement such that they rely on a jam fit between the metal shell and the patient's acetabulum. Often these metal shells have outer surfaces or coatings which encourage bone to grow into them over time. With this type of prosthesis, the polyethylene liner unit is snapped or screwed into the metal shell after the metal shell has been seated in the acetabulum to form the socket part of the joint.
Prosthetic acetabular cups generally require the use of an insertion tool to achieve correct positioning of the prosthesis in the patient's pelvic cavity. Cups which rely on a jam fit require a greater force to be applied via the insertion tool than is the case with cemented polyethylene cups. This force is usually a direct impact into the acetabulum, but force may also be applied to adjust the angular position of the cup or to remove the cup if it has been positioned incorrectly.
In order that the required forces are accurately and safely applied to the cup, it is generally necessary that the insertion tool positively grips the cup. However, it is also important that the means by which the tool grips the cup does not impinge upon the outside of the metal shell in order that in use the insertion tool does not become trapped between the shell and the pelvic bone. Further, as the wall thickness of the shell is generally kept to a minimum, the tool cannot generally grip the wall. Insertion tools are therefore generally designed to grip on a mechanical feature provided on the inner hemisphere of the metal shell. This feature is usually designed so as to cause minimum compromise to the function of the prosthetic hip joint. As a result it is often not strong enough for the impaction forces applied which may result in damage to the insertion tool, the metal shell or both.
As acetabular cups are available in a range of sizes, the tools conventionally used to insert them must similarly be provided in a range of sizes such that they can correctly fit and engage with the features provided on the cups. Having to purchase a range of such tools has cost implications for hospitals.
A third category of prosthetic hip joint exists which is manufactured entirely from metal so that the prosthetic articulation comprises a metal on metal joint. These are usually implanted without cement, relying on a jam fit in the acetabulum. With this type of cup the inner hemisphere is not a convenient place to locate a mechanical feature on which the insertion tool could grip. First, the presence of any mechanical feature on the inner surface would reduce the surface area of the prosthetic articulation. Secondly, it could cause damage to the highly polished surface of the metal.
A further problem which may be encountered is that the forces exerted through the insertion tool during impaction, repositioning and/or extraction may cause the wall of the acetabular cup to be distorted. This is particularly a problem when the cup being inserted, repositioned and/or extracted, is a resurfacing cup since these generally have a thinner wall thickness than standard acetabular cups.
It is therefore desirable to provide an insertion system and in particular an insertion tool for a prosthetic implant in which the attachment means between the insertion tool and the prosthesis is sufficiently robust to withstand the impaction and other forces to which it may be subjected during insertion of the prosthesis and which does not compromise the structural strength or the articulating properties of the prosthesis itself.
One solution to the problems of prior art arrangements is described in GB2323036 in which there is described a prosthetic implant which includes means for attaching a cable to the implant. The cable may secure a liner to the implant. A tool is provided which is connected to the implant by means of the cable. Where the liner is present, the connection of the prosthesis to the tool may be via the liner. In use the surgeon may provide force to the tool to cause the implant to be seated in the bone and then the tool is released. In one arrangement the cable is a continuous cable formed into several loops. Once the prosthesis is located in the desired position, it may be necessary to cut the cable to remove it from the prosthesis.
A second solution to the problems of prior art arrangements is described in EP1634552 in which there is described a cap for use with a prosthesis which is held in place by means of one or more cables and which includes means for cutting the cable and holding the cut cable such that it is removed when the cap is removed. An activation tool comprising a handle, an annular ring to activate the cutting means and an engagement formation is also described.
An alternative approach for securing a bone implant to a tool is described in U.S. Pat. No. 4,677,972. The arrangement described comprises a quick disconnect coupling which links a bone implant to an insertion tool. The coupling includes axially extending undercut teeth which are receivable in corresponding notches formed in the peripheral face of the implant. The teeth are locked in the notches by means of reciprocating slideable shim assemblies.
U.S. Pat. No. 5,904,688 describes a coupling which fits over the surface of an acetabular cup to enable it to interact with an insertion tool. The coupling includes a number of fingers which grip corresponding notches located in the annular face of the cup. Whilst this arrangement may offer certain advantages over prior art systems, it is apparent that the system is only suitable for use with a cup having a significant surface area on the annular face. A similar arrangement is described in U.S. Pat. No. 5,928,287.
An alternative proposal is set out in US 2004/0186586. In the described system, the acetabular cup is provided with angled recesses which are capable of receiving corresponding members located on an insertion and extraction tool.
Whilst these arrangements offer improvements over prior art devices, there is still a need for alternative arrangements and which are preferably usable with thin walled acetabular cups such as resurfacing cups.
The problems associated with prior art arrangements, may be overcome by providing a cap which in use with a prosthesis is held in the prosthesis by lugs suitable to engage recesses in the rim of the prosthesis and which is formed from a plurality of sectors mutually connected at a connection point by respective flanges.